1. Field of the Invention
This invention relates to a multipath noise detecting circuit and more particularly to such a circuit which is not affected by ignition noise nor by the level of the antenna input voltage.
2. Description of the Prior Art
There are known radio receivers additionally with a multipath noise detecting circuit for improvement in multipath noise and multipath distortion.
A conventional countermeasure for improviding multipath noise and multipath distortion is, for example, to extract a concerned signal from a section of a noise suppressor circuit (for instance, refer to IC circuit LA2110 produced by Tokyo Sanyo Electric Co., Ltd.) and cause the signal to interrupt the stereo noise control function (Referred to as SNC function) and the high cut control function (HCC function) of a multistereo demodulator (refer to IC circuit LA 3375 produced by the same Company) Practically the occurrence of multipath interference can be sensed by detecting a harmonic (noise of 100 KHz or higher) appearing due to it by the utilization of high pass amplifier, a noise detector, and a monostable multiple circuit dispersed in a noise suppressor circuit and on the basis of this, the SNC terminal and HCC terminal of the multistereo demodulator are brought to a lower level.
Another known countermeasure comprises subjecting the IF signal appearing when multipath interference occurs to level detection, and the AM-demodulation component of the detected signal is amplified and rectified in order to detect the multipath interference. Then the SNC terminal and HCC terminal of the multistereo demodulator are brought to a low level.
In the first technique is encountered a problem that because of the common use of the noise suppressor circuit, detection is made over too wide a band ranging from 90 to 300 KHz and thus no distinction is possible between ignition noise and multipath interference.
The second technique, though it improves the problem involved in the first technique, is disadvantageous in that antenna input voltages of high levels cause the limiter to operate resulting in a drop in the capability of detecting the AM-modulated component and in turn in unreliable operation.